Grammar JLPT Level N4 - Lesson 16

Structure of ～てよかった:

Verb［て］+ よかった
Verb［なくて］^(*) + よかった
Verb［ないで］^(**) + よかった

^(*) Mostly found in literary/written language
^(**) Mostly found in spoken language

Details about ～てよかった:

When て (or で when following ない) is paired with the past form of the い-Adjective, いい; てよかった, なくてよかった, or ないでよかった will have the meaning of 'I'm glad that (A)' (when positive), or 'I'm glad that I didn't (A)' (when negative).

やっぱり今日(きょう)来(き)てよかったね。
Actually, I'm glad that we came today, don't you think?
あの携帯(けいたい)を買(か)わなくて良(よ)かった。
I'm glad that I didn't buy that cellphone.
あの時(とき)にお前(まえ)と別(わか)れないで良(よ)かった。
I'm glad that we didn't break up that one time.

The primary difference between なくてよかった and ないでよかった in this expression, is that ないでよかった will be used far more frequently in casual speech. Comparatively, なくてよかった tends to be reserved for books, and may even be considered old-fashioned by some people.

先輩(せんぱい)、今日(きょう)の会議(かいぎ)で余計(よけい)なことを言(い)わなくて良(よ)かったですね。
Senpai, aren't you glad that you didn't say anything more at the meeting today? (A bit more formal, focuses on expressing reasons or results)
行(い)った人(ひと)皆(みんな)が風邪(かぜ)引(ひ)いたの？昨日(きのう)のパーティーへ行(い)かないでよかった！
Everyone that went caught a cold? I'm glad that I didn't go to last night's party! (A bit more casual, focus more on 'not doing' something)